In a case of a patient suffering from intractable hypertension, whose hypertension status is less likely to be improved even after the patient takes an antihypertensive drug, it is understood that the patient can expect blood pressure to be lowered by cutting or damaging a sympathetic nerve present around a renal artery and blocking neurotransmissions.
As a procedure of percutaneously cutting the sympathetic nerve of the renal artery, a technique is known in which a distal portion of a catheter (ablation device) for ablation is inserted into the renal artery so as to ablate the sympathetic nerve from the inside of the renal artery. This type of ablation device has an electrode unit in a distal end of an elongated shaft, and brings the electrode unit into contact with an inner wall of the renal artery. Thermal energy is applied to the sympathetic nerve present around the renal artery, thereby ablating the sympathetic nerve (See JP-T-2012-513873).
It is known that the sympathetic nerve is present around the renal artery. However, the sympathetic nerve irregularly runs through surrounding tissues of the renal artery. Consequently, it can be difficult to recognize which portion of the renal artery is in proximity to the sympathetic nerve. Therefore, in the procedure of cutting the sympathetic nerve, ablation is performed over the entire periphery along the inner wall of the renal artery, thereby more reliably cutting the sympathetic nerve. However, in some cases, an ablation site may undergo tissue degeneration and necrosis or swelling of a vascular wall. Accordingly, the ablation is not performed over the entire periphery of the inner wall at the same position. The thermal energy is applied while the position is moved in an axial direction of the blood vessel, that is, in a spiral shape at a predetermined interval.
In recent years, it has been suggested to use an optical coherence tomography technique in order to accurately recognize a position of the sympathetic nerve of the renal artery when the procedure of cutting the sympathetic nerve of the renal artery is performed. Specifically, for example, a method has been proposed in which a birefringence boundary between the nerve and a muscle or a nerve tissue and a connective tissue is detected using the optical coherence tomography technique so as to determine the position of the nerve of the renal artery. (See International Publication No. WO2013/0099797).
In the procedure of ablating the sympathetic nerve via a lumen wall of the renal artery as described above, the thermal energy is applied to each ablation site for a predetermined time, for example, of approximately 30 seconds to 2 minutes. An operator reads the temperature of the lumen wall of the renal artery and impedance variations between an electrode and a return electrode plate, or between the electrodes, and empirically determines whether the ablation is successful. That is, there has been no way to directly determine an ablation state of the sympathetic nerve. For example, in International Publication No. WO2013/0099797, when the procedure of cutting the sympathetic nerve of the renal artery is performed, the position of the sympathetic nerve of the renal artery can be accurately recognized. However, it is difficult to determine the ablation state of the ablation site after the inner wall of the renal artery is ablated using the thermal energy.
However, in the procedure of cutting the sympathetic nerve of the renal artery, in a case where the sympathetic nerve is insufficiently ablated, a sufficient therapeutic effect cannot be obtained. In a case where the lumen wall of the renal artery is excessively ablated, there is a possibility that complications may be developed with the increased risk. That is, whether or not the sympathetic nerve present around the renal artery is successfully ablated directly leads to the therapeutic effect. Therefore, in order to perform the procedure of cutting the sympathetic nerve present around the renal artery, it is necessary to determine the ablation state of the ablation site of the sympathetic nerve present around the renal artery.